1. Field of the Invention
The present invention provides an apparatus and a method for measuring and recording the blood pressure waveform of a patient by detecting momentary increases in the pressure of an occlusive blood pressure cuff, and recording the time and magnitude at which the pressure increases appear and disappear with respect to the R-wave of an electrocardiograph signal.
2. Prior Art
Currently used non-invasive techniques for determining the arterial blood pressure use the sphygnomanometric principle. This familiar blood pressure measuring technique generally comprises the steps of placing the occlusive cuff around the brachial portion of the arm and rapidly inflating it to a pressure substantially higher than the expected systolic pressure level. At this point, both the arterial and the venous flow to the arm distal to the cuff are occluded. Both the artery and the vein under the cuff are collapsed since the external pressure, transferred from the cuff by the tissue, is higher than the intra-vessel pressure. Typically, the maximum (systolic) arterial blood pressure at this point is 120 torr (mm of mercury), the diastolic (minimum) pressure is 80 torr, and the pressure of the returning venous blood is 15 torr. Thus, the cuff is inflated to a pressure in excess of 120 torr for a typical normotensive patient.
The cuff pressure is then slowly decreased until it reaches a point at which the peak arterial pressure is slightly higher than the occlusive cuff pressure. At this point, the intra-arterial pressure overcomes the external pressure, the artery expands, and a bolus of blood is pushed through the artery into the distal portion of the arm. The artery at this point is severely pinched; therefore, the resulting momentary flow is turbulent (normal flow is laminar) giving rise to Korotkoff sounds heard by a stethoscope placed on the brachial artery below the cuff. The pressure at which the first appearance of Korotkoff sounds is detected is generally denominated as the systolic pressure level.
As the cuff is slowly further deflated, the ratio of the time in which the arterial pressure is higher than the cuff pressure to the time in which the artery is occluded increases. As the pinching of the artery becomes less severe, the flow is less turbulent, and the Korotkoff sounds, although longer in duration, are less pronounced until they completely disappear. At this point, the artery is substantially fully expanded and the arterial flow is laminar. The pressure at this point is generally denominated as the diastolic pressure level.
Since the brachial vein is still collapsed at cuff pressures below the diastolic pressures and above the venous blood pressure, the flow to that portion of the arm distal to the cuff results in blood pooling, felt by the patient as a discomfort or pain. The venous pool will not be released until the cuff pressure falls below the venous pressure level. For that reason, when there is an assurance that the diastolic pressure level has been passed, the cuff is rapidly deflated to minimize the discomfort to the patient.
The auscultatory method described above has built-in limitations of accuracy since it depends upon a human operator. For instance, the person performing the blood pressure check may fail to hear or recognize the auditory changes associated with the two pressure levels. Furthermore, the blood pressure may be decreased too rapidly to obtain an accurate reading of pressure at which the sounds change.
The need to reduce the human error in obtaining blood pressure measurements, and the need for automatic periodic monitoring of blood pressure has resulted in the development of semiautomatic and fully automatic blood pressure measurement systems. Generally, these systems essentially emulate the above-described method of measurement. The difference is typically in the method of detecting the systolic and diastolic pressure levels. In one prior method, a microphone is built into the cuff and replaces the stethoscope used by the physician or other person performing the measurement. An electronic device connected to the microphone will detect the appearances and disappearances of the Korotkoff cuff sounds and correlate them with the cuff pressure when the changes are detected. The accuracy of this method will be limited by the ability of the electronic circuitry to discriminate between the turbulent blood flow and any other sounds that may be present. In many instances, the tests are performed in a soundproof room to eliminate extraneous noise and obtain a desired degree of accuracy. Furthermore, the apparatus requires an electrical connection between the cuff and the system in addition to the pneumatic connection to supply pneumatic pressure to the cuff.
Another method developed is the use of two pressure transducers placed at the beginning and end of the occlusive cuff. The two transducers detect the differential pressure caused by the propagating bolus of blood and therefore can detect the beginning of turbulent blood flow. Again, an electrical connection is required to connect the pressure transducers to the monitoring system.
One of the simplest fully automated methods of detecting blood pressure is the oscillometric method. The oscillometric method only requires one transducer, which can be located at the system end of the pneumatic connection between the cuff and the system. Thus, no electrical connections are required between the patient and the system. The oscillometric method detects the momentary increases in cuff pressure caused by the passage of a bolus of blood through the artery beneath the cuff. The momentary expansion of the artery results in an increase in the volume of the tissue under the cuff and is reflected in a momentary increase of the cuff pressure. As the cuff pressure becomes lower, the volume of the bolus of blood is higher and the momentary increase of the cuff pressure is higher.
The amplitude of the pressure pulses reflected back into the cuff pressure increase to a point at which the mean pressure level is reached. From this point, their magnitude will progressively decrease until the diastolic pressure level is passed, from which level they will stay at a constant amplitude level dictated by the arterial pressure compliance. The first appearance of the cuff pressure pulses defines the systolic pressure level, the maximum amplitude of the pulses corresponds to the mean arterial pressure level, and the last measurable decrease of the amplitude of the cuff pressure pulses signifies the diastolic pressure level.
The oscillometric method has a number of drawbacks including the fact that only the systolic, diastolic and mean pressure levels can be determined. More importantly, the shape of the arterial pressure waveform, which has significant diagnostic and clinical meaning, is unknown. The maximum rate of systolic pressure change, an indicator of the heart's contractility, cannot be determined. Finally, there is no verification that measured arterial pressure levels are correct since the transducer is attempting to measure a transient pressure level.
The accuracy of the pressure measurement is indirectly proportional to the rate of deflation of the cuff. An increase in accuracy obtained by reducing the deflation rate results in a corresponding increase in the pain and discomfort to the patient because of blood pooling. Thus, there is a need for an automated blood pressure measurement method and device to increase the accuracy of the measurements, to provide more diagnostic and clinical information, and to reduce the pain and discomfort to the patient.